直到前不久，准确的妊娠期还是根据末次月经日期、子宫尺寸和胎儿在母腹中最初微动的时间来确定的。但是，由于不可能准确确定排卵时间，就大大降低了上述种种根据在临床实践中的运用。1973年，Robinson为了确定（妊娠）时间，在妊娠的头三个月中利用超声波仪器测量胎儿骶骨一顶骨的尺寸，据此确定在妊娠期与胎儿骶骨一顶骨尺寸之间的相互关系。 Until recently, accurate gestational age was also based on the last menstrual date, uterine size, and the time the fetus initially fretted in the womb. However, since it is impossible to accurately determine the time of ovulation, the above-mentioned basis is greatly reduced in the clinical practice. In 1973, Robinson determined the size of the parietal sacral apex by ultrasound during the first trimester of pregnancy in order to determine the timing of (gestational) pregnancy, which determines the interrelationship between gestational age and the size of the fetal sacral apex.